Incorporating Mullins Effect into the Modeling of Hyperelastic Actuators

Abstract

The use of hyperelastic materials in actuators has become increasingly popular due their immense number of degrees of freedom, ability to recover from large deformations, and the capability to absorb energy in many types of environments. Some prototypes for these actuators utilize instabilities that result from the nonlinear stress-strain response in these hyperelastic materials. However, these prototypes fail to include in their supporting analysis any sort of irreversible processes whose inclusion would help realize their potential in cyclical loading scenarios. One such irreversible framework is the Mullins effect, which describes a range of phenomena, most importantly stress softening, that all depend on the hysteresis in deformation of the material. This thesis proposes a comprehensive model that captures Mullins effect stress softening and its subsequent convergence.

An experimental study was performed to calibrate a hyperelastic pressure-volume relationship for thin-walled cylinders with Ogden-Roxburgh damage. Included in this model is a fractional multiplier to capture the convergence of stress softening after roughly 10 cyclic loadings. The Ogden-Roxburgh model was calibrated using uniaxial tension tests on super-soft latex samples, and the fractional multiplier was calibrated via cyclic fluid loadings on separate samples. Ultimately, this model allows for a more precise understanding of how a hyperelastic actuator will respond over many load cycles.